The petroleum industry generally classifies petroleum waxes in three main categories, namely (1) paraffins, (2) intermediate, and (3) microcyrstallines. Other classes of waxes include the scale waxes and slack waxes. Generally, in a commercial petroleum process the paraffin waxes comprise the 90, 200 and 350 distillate fractions from a vacuum distillation unit. These numbers represent Saybolt universal seconds, a measure of the distillate viscosity at 100.degree. F. This wax is a mixture of solid hydrocarbons derived from the overhead wax distillate fraction obtained from the fractional distillation of petroleum crude oils. The paraffin wax, after purification, is a substantially colorless, brittle and low viscosity material normally having a melting point of from about 120.degree. F. to about 160.degree. F.
The heavy intermediate waxes are produced from the 650 distillate wax fraction of a vacuum distillation unit, e.g., Saybolt Universal seconds at 100.degree. F. The temperature and pressure of the vacuum residue are optionally adjusted to distill off the 650 distillate wax fraction in the process herein. It should be noted that petroleum crude oils from different sources will have different optimum temperatures for distilling off the 650 distillate wax fraction. These heavy intermediate waxes have a melting point of from about 145.degree. F. to about 185.F. and exhibit somewhat different physical properties than the individual paraffin waxes and microcrystalline waxes.
Microcrystalline waxes are conveniently produced from the nondistillable vacuum tower residues or resids from the fractional distillation of petroleum crude oils. These waxes differ from paraffin waxes in having branched hydrocarbons of higher molecular weight. They are considered more plastic than paraffin waxes, normally are dark colored or opaque, and usually have a melting point of from about 150.degree. F. to about 200 F.
The categories of waxes described are obtained as by-products in the manufacture of lubricating oils. They are used for a number of purposes, such as, in the coating of a variety of substrates, in hot melt adhesives, in the manufacture of candles, and the like. Prior to such uses, they are ordinarily subjected to various finishing processes, such as bauxite or fullers earth percolation, hydrotreating, etc. to remove oxygenates, olefins and aromatic color causing compounds. The various categories of waxes are used alone or in combination in formulations which may include a variety of polymers, resins, anti-oxidants and other additives.
In the manufacture of hot melt adhesives, the desirable wax component has a high melting point and a relatively narrow melting range which offers good compatibility with the polymers and resins used in such adhesives. Heavy intermediate waxes have the requisite melting points but are usually of much wider melting range than is preferred for hot melt adhesives. Such waxes can be fractionated by conventional vacuum distillation to provide narrow melting range fractions, however, the temperatures required usually result in fractions containing undesirable degradation products which adversely affect the properties of the wax, in particular the color and odor.